Millions of tons of volatile organic compounds (VOCs) enter and pollute the environment each year. In particular, this pollution occurs as a result of improper disposal or treatment of industrial wastewater often resulting in contaminated groundwater. Although contaminated groundwater frequently only contain parts per billion levels of VOCs, this level of contamination is sufficient to render water unfit for human consumption. Cleanup of contaminated groundwaters is difficult, expensive and time-consuming.
Conventional treatment methods for removal and recovery of VOCs include air stripping, adsorption, advanced oxidation and distillation, are often effective only at specific VOC concentration levels and may result in farther environmental contamination such as the release of gaseous VOC emissions.
Pervaporation is a membrane based separation process which overcomes many of the deficiencies of various technologies for the recovery/removal of organic compounds from industrial waste streams or contaminated water in the environment. In pervaporation involving organic and aqueous components, the organic compounds in a liquid stream may be driven across a permselective membrane emerging as a vapour on the downstream side of the membrane which may be subsequently condensed and collected. A vacuum pressure maintains the separation driving force. Permeation of organic compounds is governed by the properties of the membrane. Normally, membranes having dominant hydrophobic properties permit the permeation of organic compounds while preventing the permeation of water. Similarly, ultrafiltration is another membrane based separation process which overcomes deficiencies in the recovery/removal of organic compounds from industrial waste streams. In ultrafiltration, separation occurs through the membrane wherein the upstream and downstream components are both in the liquid phase.
In the alternative, a membrane having dominant hydrophilic properties will permit the permeation of water through the membrane thereby effecting a separation of aqueous and VOC components. Generally, membrane characteristics will dictate whether a membrane is suitable for the extraction of VOCs from aqueous solutions or for the extraction of water.
For example, membranes made from hydrophobic elastomeric polymers have been known to be highly organophilic and effective in VOC removal from water by pervaporation. To date, the most practical pervaporation applications have been reported with polydimethylsiloxane (PDMS), ethylene/propylene rubber (EPR) or polyether-block-amid (PEBA) membranes. PDMS composite membranes for the removal and recovery of trace organics from water are commercially available. PDMS membranes, however, require chemical treatment to control swelling which results in the reduction of both permeation rate and selectivity due to the absorption of feed components.
Membrane efficacy requires large contact surface areas. PDMS composite membranes, however, due to weak mechanical properties require specially designed modules in order to provide a high surface to volume ratio and, hence, are limited with respect to their industrial applicability.
In the particular application of pervaporation, many hydrophobic membranes used for pervaporation are limited with respect to their mechanical stability, as indicated above, or with respect to their selectivity to be useful in particular separations. Furthermore, many hydrophobic membranes are of limited use because of a tendency to foul thus resulting in unacceptable permeation rates.
Accordingly, there has been a need for a class of membranes which overcome the deficiencies as described above and specifically, there has been a need for a class of membranes which preferentially separate water from an aqueous/VOC mixture.
Specifically, there has been a need for a class of membranes in which the bulk hydrophilic properties of the membrane are not completely dominated by the hydrophobic surface properties of the membrane so as to remain hydrophilic, that is to allow the permeation of water, while remaining hydrophobic at the surface, to prevent swelling of the hydrophilic membrane underneath. Furthermore, there has been a need for a class of membranes with the above properties which further minimize membrane fouling.
A review of the prior art shows that hereto before, the design of membranes for pervaporation having high surface hydrophobicity which enable the permeation of water has not been achieved.
For example, Fang et al (Effect of Surface Modifying Macromolecules and Solvent Evaporation time on the Performance of Polyethersulfone Membranes for the Separation of Chloroform/Water Mixtures by Pervaporation, Journal of Applied Polymer Science, vol 54, 1937-1943, 1994) describes the preparation and testing of SMM modified membranes in the separation of chloroform/water mixtures wherein organics preferentially permeate through the membrane.
Pham et al. (Design and Development of a Polymeric Additive to Enhance the Performance of Polyethersulfone Membranes for the Removal of Organic Compounds from Aqueous Solutions by Pervaporation, Chemical Engineering Society Abstract, October 1994) describes removal of volatile organic compounds from aqueous solutions. Fang et. al. (Application of Surface Modifying Macromolecules for the Preparation of Membranes with High Surface Hydrophobicity to Extract Organic Molecules from Water by Pervaporation, Proceedings of Seventh International Conference on Pervaporation Processes in the Chemical Industry, Reno, Nev., February 1995) describes the preparation of SMM modified membranes in which organics permeate through the membrane.
Anh, Pham (Surface Modifying Macromolecules for Enhancement of Polyethersulfone Pervaporation Membrane Performance, University of Ottawa, Department of Chemnical Engineering, August 1995) describes the preparation of membranes containing SMM useful in the separation of water/organics wherein organics are removed from a solution containing volatile organic compounds.
PCT publication WO 90/11820 describes a porous article having a hydrophobic polymer body with integrated hydrophilic polymer co-precipitated with the hydrophobic polymer. Canadian Patent Application 2,051,497 describes a membrane process in which isopropanol is recovered by pervaporation across a membrane of polyethylene imine. Canadian Patent Application 2,015,413 describes a process for preparing a microporous membrane having both hydrophilic and hydrophobic polymers. Canadian Patent Application 2,138,056 describes a hydrophilic polytetrafluoroethylene membrane used in the pharmaceutical and medical industries. Canadian Patent Application 2,046,332 describes a composite membrane for separating water from fluids containing organic compounds by pervaporation, the composite membrane having cross-linked polyvinyl alcohol. Canadian Patent Application 2,115,648 describes an osmotic drug-delivery device having hydrophobic wall materials.